The present invention relates to a circuit for monitoring a high voltage and, more particularly, to a voltage level detection circuit that has the ability to limit the input current to a predetermined value and thereby limit the power dissipated despite an increase of the high voltage being monitored.
There are a wide variety of circuits available to monitor or detect the level of an input voltage. Many of these conventional voltage level detection circuits use a resistor voltage divider as a basic building block. The voltage detection circuit compares the voltages at nodes along the voltage divider against a reference voltage that can be derived by means of a transistor chain.
This type of voltage detection circuit is subject to various limitations, one of which is the relatively high level of power dissipated in the resistive voltage divider. This power is wasted and thus reduces the overall efficiency of the voltage detection circuit. Another disadvantage of the resistor voltage divider type detection circuit is that the current dissipated will increase with the level of the input voltage being monitored.
A voltage level detection circuit of this type is described in the U.K. Patent Application, GB 2030398A in the name of Musa et al. This published patent application utilizes a constant voltage reference generator consisting of three field effect transistors connected in series across the terminals of a DC voltage supply whose voltage level is to be monitored. The voltage reference generator provides a constant output voltage which does not vary with variations of the monitored power supply voltage. A resistor voltage divider is also connected across the terminals of the DC power supply voltage. Three comparator circuits each have one input connected to the output of the voltage reference generator and second inputs connected to respective nodes along the resistor voltage divider. The voltages at these nodes vary with the level of the DC supply voltage. The comparators produce control voltage signals when the voltages sensed by the resistor voltage divider reach predetermined levels. This voltage protection circuit is subject to the disadvantages enumerated above, i.e. excessive current and power dissipation which increases as the monitored input voltage increases. In addition, the trip point voltage of the detection circuit is difficult to control.